Stephen Biegun / AP

The U.S. envoy for North Korea called on the regime Thursday to provide a full declaration of its nuclear and missile programs as a second bilateral summit appeared only weeks away.

Special Representative Stephen Biegun, who is in charge of day-to-day negotiations with the North, said the declaration will be required to acknowledge complete denuclearization of the regime.

"Before the process of denuclearization can be final, we must also have a complete understanding of the full extent of the North Korean weapons of mass destruction missile programs. We will get that at some point through a comprehensive declaration," Biegun said during a speech at Stanford University.

He said the U.S. has "contingencies" prepared in case negotiations fail, but did not give details.

North Korea has balked at such a demand, claiming an inventory will give the U.S. a list of targets to destroy.

Biegun made clear the U.S. also intends to verify denuclearization.

"We must reach agreement on expert access and monitoring mechanisms of key sites to international standards, and ultimately ensure the removal and destruction of stockpiles of fissile material, weapons, missiles, launchers, and other weapons of mass destruction," the envoy said.

The U.S. and North Korea reached a vague denuclearization deal during last June's first summit between U.S. President Donald Trump and North Korean leader Kim Jong-un.

The agreement committed the North to work toward complete denuclearization of the Korean Peninsula in exchange for security guarantees for Pyongyang, without laying out concrete steps.

In late February, the two leaders are set to meet again, reportedly in Vietnam, to try to flesh out the deal. Trump said earlier Thursday that the exact date and venue will be announced early next week.

Many North Korea watchers in the U.S. have warned that the two sides will need to agree on a detailed implementation plan before the second summit or risk giving the regime the legitimacy it seeks as a nuclear weapons state.

Biegun said U.S. Secretary of State Mike Pompeo was promised by Kim during his October trip to Pyongyang that the regime would dismantle and destroy all of its plutonium and uranium enrichment facilities -- not just those in the main nuclear complex in Yongbyon -- in exchange for "corresponding measures" from the U.S.

"In describing to us their commitment to dismantle and destroy their plutonium and uranium enrichment facilities, the North Koreans have also added the critical words 'and more,'" the envoy said. "This is essential, as there is more -- much more -- to do beyond these facilities to follow through on the Singapore summit commitment to complete denuclearization."

He also said that the U.S. has told the North Koreans that it is prepared to carry out the commitments from Singapore's first summit "simultaneously and in parallel."

The offer is in line with North Korea's wishes as the regime has accused the U.S. of making "gangster-like" demands to first surrender all of its nuclear weapons for only a promise of future economic concessions.

Biegun noted that the U.S. has eased rules for the delivery of humanitarian aid to North Korea, which was previously delayed by United Nations sanctions on the regime.

But he reiterated that the U.S. will keep punishing sanctions on the North until it sees complete denuclearization.